1. Field of the Invention
The present invention relates to a method for producing a pigment, and particularly, to a method for producing a pigment represented by a metal phthalocyanine, which is useful as a coloring material and for use in electrophotograph and other fields, with high yield and in an environmentally friendly process (hereinafter, referred to as a “green process”).
2. Description of the Related Art
As a conventional method for producing a pigment, in particular, a metal phthalocyanine, the Wyler method and the phthalonitrile method have been most frequently employed in the industrial field.
In the Wyler method, a pigment is synthesized by heating phthalic anhydride, a metal compound and urea in an inert solvent in the presence of a catalyst. On the other hand, in the phthalonitrile method, a pigment is synthesized by heating a phthalocyanine and a metal or a metal salt in a solvent.
The Wyler method has the following problem. The reaction does not proceed uniformly because all of a raw-material mixture, reaction intermediate and product are not completely dissolved in a reaction process. Consequently, the product is not obtained with high purity or in high yield. In addition, since an organic solvent such as an alkylbenzene is used as a reaction solvent, this method is not desirable in view of environmental load.
Unlike the Wyler method, in the phthalonitrile method, a metal phthalocyanine can be synthesized with high purity and in high yield. However, the reaction must be performed at a sufficient high temperature and in a solvent, which is capable of dissolving raw material such as phthalonitrile and a reaction intermediate sufficiently. Therefore, a solvent such as an alkylbenzene having a high-boiling point is often used. However, it is not desirable to use such a solvent because separation of the solvent from a product is difficult and environmental load increases.
Japanese Patent Application Laid-Open No. H07-316448 is concerned with the Wyler method and discloses that a product can be obtained in high yield by adding a predetermined additive in a reaction process to disperse raw materials, thereby increasing a reaction rate. On the other hand, Japanese Patent Application Laid-Open No. H05-179153 is concerned with the phthalonitrile method. In this method, a low boiling-point alkylbenzene having a branched C6 side chain is used as a solvent. By virtue of this, the publication of Japanese Patent Application Laid-Open No. H05-179153 discloses that a product can be obtained in high yield by improving the solubility of the raw materials, thereby increasing a reaction rate, and that the solvent can be removed from the product by a simple operation such as removal under reduced pressure.
Nevertheless, in both methods disclosed in Japanese Patent Application Laid-Open Nos. H07-316448 and H05-179153, there is room for improvement. More specifically, an environmentally unfriendly organic solvent, such as an alkylbenzene used as a reaction solvent, is susceptible to improvement. In the circumstances, it has been desired to develop a method for synthesizing a high-purity metal phthalocyanine with high yield and in a green process.
Note that the term “environmentally friendly process” in the present invention means a process that does not discharge harmful wastewater and the like causing environmental pollution as much as possible.